Interactions of RadB, a DNA repair protein in archaea, with DNA and ATP.

The RecA family of recombinases (RecA, Rad51, RadA and UvsX) catalyse strand-exchange between homologous DNA molecules by utilising conserved DNA-binding modules and a common core ATPase domain. RadB was identified in archaea as a Rad51-like protein on the basis of conserved ATPase sequences. However, RadB does not catalyse strand exchange and does not turn over ATP efficiently. RadB does bind DNA, and here we report a triplet of residues (Lys-His-Arg) that is highly conserved at the RadB C terminus, and is crucial for DNA binding. This is consistent with the motif forming a "basic patch" of highly conserved residues identified in an atomic structure of RadB from Thermococcus kodakaraensis. As the triplet motif is conserved at the C terminus of XRCC2 also, a mammalian Rad51-paralogue, we present a phylogenetic analysis that clarifies the relationship between RadB, Rad51-paralogues and recombinases. We investigate interactions between RadB and ATP using genetics and biochemistry; ATP binding by RadB is needed to promote survival of Haloferax volcanii after UV irradiation, and ATP, but not other NTPs, induces pronounced conformational change in RadB. This is the first genetic analysis of radB, and establishes its importance for maintaining genome stability in archaea. ATP-induced conformational change in RadB may explain previous reports that RadB controls Holliday junction resolution by Hjc, depending on the presence or the absence of ATP.